The present invention relates to electrographic recording. More particularly, it relates to an improved recording apparatus using electrographic techniques.
Heretofore there has been a well-established technology relating to a form of electrographic recording. In according with that technology, a latent electrostatic image is formed on a record carrier. The electrostatic image is then developed by the deposition of a colored toner thereon which adheres to the carrier in the pattern of the latent image. The toner image is then fixed as by heating or pressure to form a permanent record.
More recently, there has been developed an improved electrographic recording technology wherein the need for providing a latent electrostatic image is eliminated. That technology is illustrated in Kotz U.S. Pat. Nos. 3,816,840, Lunde 3,879,737, and Lunde 3,946,402. In accordance with the teachings in those patents, a quantity of toner powder which has both electrical and magnetic properties, is contained in a hopper positioned adjacent a drum made of non-magnetic material. Within the drum there is a magnet structure creating magnetic fields extending through the periphery of the surrounding drum. In one form of the structure disclosed the drum rotates about an axis coaxial with the central magnet structure while the magnetic core is fixed. In another form, the outer drum is stationary while the inner magnetic core is rotated. In either case, the toner powder is drawn from the hopper by the magnetic fields to be attracted to the outer periphery of the drum. The rotation of the drum or the magnetic core causes the toner particles to be translated around the periphery of the drum to a recording station.
At the recording station, a recording medium comprising a backing sheet such as paper having a dielectric coating thereon is driven along a path spaced a small distance from the surface of the toner bearing drum. An electrically conductive element is positioned on the opposite side of the record member from the drum. Under the influence of the magnetic fields, the toner particles form whisker like strings about the surface of the drum. These whisker strings are of sufficient length to brush the surface of the record member. Because of the magnetic fields, these toner particles are not deposited upon the surface of the record receiving member unless an electric field is established between the drum or electrodes carried by the drum and the backup conductive plate on the opposite side of the record member. When such electric fields are established the electrostatic charges over power the magnetic field influence and deposit the toner particles in selected areas on the record member in accordance with the pattern established by the electric fields.
In an analogous embodiment shown in the aforementioned patents, a somewhat different technique is employed in that the record member is uniformally coated with the toner powder and the unwanted portion of the toner powder is than picked off the magnetic field, the desired portion being adhered to the record member by the superposition of an electric field in the manner aforementioned. These improvements have thus provided a recording technique wherein records of data may be made without first imposing an electrostatic image on the record member which must then be developed by the toner. It is, in effect, a direct writing technique.
There are, however, certain disadvantages in the techniques described which have been observed. In the technique wherein the outer drum is held stationary and the inner magnetic core is rotated to produce the movement of the toner about the periphery of the drum, the speed of writing on the record member is limited by the rotational speed of the magnetic structure, it being necessary for a magnetic pole to be present at the writing station at the instant of pulsing the electrodes as is shown in U.S. Pat. No. 3,914,771--Lunde et al. In the technique wherein the recording medium is uniformly coated with the toner and the toner selectively removed by the conjoint operation of the electric and magnetic fields, it has been found that the background portion of the record thus produced is darkened by the residual toner not completely removed by the magnetic field. This produces a reduced contrast record. In the embodiment wherein the magnetic field is stationary and the outer drum is rotated, the signal electrodes must be those positioned on the reverse side of the record member. This arrangement has not been found to produce a satisfactorily sharp record.